1. Field of the Invention
The present invention relates to a semiconductor device that comprises a semiconductor substrate and an insulated gate field-effect transistor (MISFET) provided on the substrate, and to a method of manufacturing the semiconductor device. The invention relates to the gate electrode of a MISFET, which is made of metal, and to a method of manufacturing the gate electrode.
2. Description of the Related Art
In a method of manufacturing a MOSFET whose gate electrode is made of metal, the metal gate is formed by damascene metal-gate process. In the damascene metal-gate process, the trench made by removing a dummy gate is filled with metal. The smaller and narrower the metal gate of the MOSFET, the more difficult it is to fill the trench with metal. Therefore, there is a demand for a MOSFET structure and a method, with which damascene metal-gate process can be successfully performed even if the metal gate is small and narrow.
FIGS. 1A and 1B are sectional views, explaining damascene metal-gate process of forming the gate electrode of a conventional MOSFET.
In this damascene metal-gate process, a dummy gate (not shown) is removed, making a trench 90 in an inter-layer insulating film 95 as shown in FIG. 1A. Thereafter, as FIG. 1B shows, metal 96 is deposited on the film 95 and in the trench 90. In FIGS. 1A and 1B, numeral 91 designates a dummy-gate insulating film, numeral 92 indicates a silicon nitride film that surrounds the trench 90, numeral 93 denotes a gate side-wall (silicon oxide film), numeral 94 represents a silicon nitride film, and numeral 97 denotes a gate insulating film.
The MOSFET is so small that its channel length is, for example, 30 nm and the trench 90 has a height of, for example, 220 nm, as is illustrated in FIGS. 1A and 1B. The trench 90 therefore has a very large aspect ratio. Due to the large aspect ratio, the trench 90 cannot be completely filled with metal 96 if the metal 96 is deposited by sputtering. Consequently, a space S is made in the trench 90.
To fill the trench 90 with metal 96 fully and completely, the height of the dummy gate may be decreased. However, the height of the dummy gate cannot be easily reduced, because it is also used the dummy gate as a polishing stopper of the inter-layer insulating film 95 by chemical mechanical polishing (CMP).
If the trench 90 is made narrower so that it may be filled with metal 96 completely, the metal 96 will form a metal gate that has but a small cross section. The metal gate inevitably has an excessively high resistance, jeopardizing the high performance of the MOSFET. The trench 90 may be broadened. If the trench 90 is broadened, however, the MOSFET will have a long gate length, which also impairs the performance of the MOSFET.
As described above, the conventional damascene metal-gate process is disadvantageous because it is difficult to fill up a trench with metal to form the metal gate of a MOSFET particularly when the metal gate is small and narrow. If the trench is made shallow so as to be completely filled with metal, the resultant meal gate will have a high resistance, jeopardizing the high performance of the MOSFET. If the trench is broadened for the same objective, the gate length will increase, inevitably impairing the performance of the MOSFET.
According to one aspect of the present invention, there is provided a semiconductor device semiconductor device which comprises: a semiconductor substrate; at least one pair of impurity-diffused layers formed in a surface of the semiconductor substrate, isolated from each other and constituting a drain region and a source region of an insulated gate field-effect transistor; a channel region provided between the drain region and the source region; a gate insulating film provided on the channel region; a gate electrode including a lower part and an upper part, the lower part having a lower surface and sides, the upper part having a lower surface, the lower surface of the lower part contacting the gate insulating film, and the upper part being longer than the lower part in a lengthwise direction of the gate electrode; and a first insulating film interposed between the lower surface of the upper part of the gate electrode and the semiconductor substrate, surrounding at least the sides of the lower part of the gate electrode, which face the drain and source regions, and having parts located adjacent to the sides of the lower part of the gate electrode and having a height greater than a thickness of the other parts located between the lower surface of the upper part of the gate electrode and the semiconductor substrate.
According to another aspect of the invention, there is provided a method of manufacturing a semiconductor device, the method comprises: preparing a semiconductor substrate; forming a dummy-gate insulating film and a dummy gate having a circumferential surface, on a region of the semiconductor substrate, in which an insulated gate field-effect transistor will be formed; introducing an impurity into a surface of the semiconductor substrate selectively, thereby forming a drain region and a source region of the insulated gate field-effect transistor and forming a channel region between the drain region and the source region; forming a first insulating film on the circumferential surface of the dummy gate, the first insulating film having a predetermined etching ratio to the dummy gate; forming a second insulating film above the semiconductor substrate, the second insulating film having a predetermined etching ratio to the first insulating film; processing the second insulating film, thereby giving a substantially flat surface thereto and burying the dummy gate and the first insulating film in the second insulating film; removing a part of the first insulating film to leave the remaining part of the first insulating film at a prescribed height, the remaining part contacting the circumferential surface of the dummy gate; removing the dummy gate and the dummy-gate insulating film above the channel region, thereby making a trench surrounded by the second insulating film and the first insulating film; forming a gate insulating film on the channel region; and filling the trench with metal, thereby forming a gate electrode.